Fluxing petroleum coke slag



United States Patent OfiFice 2,835,213 Patented May 20, 1958 FLUXING PETRGLEUM CDKE SLAG Davis A. Skinner, Fullerton, Califl, assignor to Union Oil Company of California, Los Angeles, Calif., a corporation of California No Drawing. Application January 29, 1952 Serial No. 268,915

7 Claims. (Cl. 110-1) This invention relates to the fiuxing of petroleum coke slag, and in particular concerns a method for lowering the flow temperature of the slag produced by burning petroleum coke.

In the refining of petroleum it is common practice to subject heavy crudes or heavy residual fractions to a socalled coking operation whereby the heavy oils are thermally cracked to form light oils and a highly carbonaceous solid material which is known as petroleum coke. The latter product has considerable heating value and is widely used in refineries and elsewhere as boiler fuel, usually in pulverized form in the same manner and in the same type of boiler as pulverized coal. In the cornbustion of petroleum coke, the non-combustible and nonvolatile components of the coke accumulate in the firebox in the form of a glass-like slag comprising silica, alumina and minor amounts of other metallic oxides. Such slag is deposited in various locations within the boiler, e. g. on the firebox fioor, on the boiler tubes, etc., and samples of the slag taken from different locations usually vary somewhat in appearance and chemical composition. In order to maintain high boiler efliciency it is necessary that the slag be removed from the firebox, and in order to provide for this most fireboxes are of the so-called wetbottom construction wherein a tap is provided in thefloor of the firebox for the purpose of drawing off the slag in liquid form. In a number of instances, however, the composition and nature of the slag is such that it remains solid, or at least does not readily flow, at the temperature of the firebox floor so that it cannot be drawn off through the tap in the customary manner. in such case, it becomes necessary periodically to shut down the boiler and to remove the accumulated slag manually. Such procedure is undesirable and uneconomical from the standpoint of continuous and emcient operation of the boiler, and is particularly onerous because of the unusual hardness of the glass-like slag which adheres tightly to various portions of the firebox.

It is accordingly an object of the present invention to provide a means for removing the slag from boilers and the like which operate on petroleum eokefuel.

Another object is to provide means whereby slag is prevented from accumulating in boilers which employ petroleum coke as fuel.

A further object is to provide means for lowering the flow temperature of petroleum coke slag to such a value that the slag is relatively fluid at the temperatures which prevail at or near firebox floors.

Other objects will be apparent from the following description of the invention, and various advantages not referred to specifically herein will be apparent to those skilled in the art upon employment of the invention in practice.

I have now found that the above and related objects may be realized through the use of sodium fluoride as a fiuxing agent for the slag. More particularly, I have found that by mixing sodium fluoride with petroleum coke slag the temperature at which the slag becomes readily flowable may be reduced to a value below that of the prevailing temperature at or near the firebox floor, thereby permitting the slag to be drawn off through the tap hole of a wet-bottom furnace and recovered from the firebox without necessitating a shutdown and its associated difficulties. The invention thus consists essentially in introducing sodium fluoride into the firebox in such manner that it comes into contact with the slag deposited therein, thereby fluxing the slag and permitting its removal from the firebox in substantially liquid form.

The standard method for determining the flow point of slags is known as ASTM Method D271-40, and is described in ASTM Standards, 1940 Supp. Part III, pp. 19-22. According to the method, the slag is pulverized and formed into a cone with the aid of a dextrine solution binding agent. The cone is then mounted on a refractory base plate and, after ignition to remove the carbonaceous material, is placed in a furnace and is heated at a uniform rate. The temperature at which the cone fuses down to a spherical lump is taken as the softening temperature and the temperature at which the lumps spread out flat on the refractory base is taken as the fluid temperature. While such test method is satisfactory insofar as determining the softening and flow temperature is concerned, it does not provide satisfactory information on the viscosity of the molten or semi fluid slag. To provide such information. the method has been modified as follows: The cone is molded in the standard manner, but the apex is cut off so that the height of the cone is 0.5 inch. The cone is then placed upside down in a 0.75 inch length of 0.5 cm. ID refractory tubing which is mounted in the refractory base plate. A small slot is cut in the tube to provide a gas vent. The size of the cone with respect to the tube is such that it hangs inside the tube with about 0.25 inch protruding from the top. The assembly is placed in a furnace and heated in the standard manner. The temperature at which the visible ,portion of the cone disappears inside the tube is taken as the down temperature. Heating is continued until a predetermined top temperature is reached, after which the assembly is removed from the furnace and is allowed to cool. The tube is then sawed in half lengthwise, and the distance the melted slag has flowed down the inside of the tube is measured. From such measurement it is possible to evaluate the flow characteristics of the melted slag at the top temperature, and to estimate the ease with which the melted slag will flow through a relatively small hole such as the tap hole in the floor of a wet-bottom furnace. The following table presents a comparisonof the ASTM softening and fluid temperatures with the down temperature on samples of three different petroleum coke 'slags:

1 Determination made uncertain by bubbling of melted sample.

At a predetermined top temperature of 2377 F., Sample No. 1 ran down the inside of the tube a distance of 8 mm. and showed no evidence of wetting the wall of the tube.

The following table presents experimental data on the fiuxing effect of various inorganic fluxes on a typical petroleum coke slag. Such slag was a black glass-like material which had been removed from the firebox floor of a boiler operating on petroleum coke "obtained by coking a Santa Maria Valley, California crude oil. It had a hardness of 7 (Mohs scale), exhibited conchoidal fracture, and resembled obsidian in appearance. X-ray examination showed it to contain crystalline spinels, and spectrographic analysis indicated the presence of major amounts of sodium and silicon, moderate amounts of iron, potassium, calcium, nickel and aluminum, and small amounts of magnesium, manganese, chromium, titanium, vanadium and other metallic elements. Chemical analysis showed the presence of 56.8 percent of silica and 14.0 percent of alumina- In testing the efficiency of the various fluxing agents, the particular agent was admixed with the powdered slag in the indicated amount, and cones were molded from the mixture as above described. The inverted cone method of testing was employed. The temperature at which the cone disappeared into the tube was taken asthe down temperature, and the cones were further heated to a pre-selected top temperature. After cooling, the tubes weresawed in half, and the distance of flow of the melted slag down the inside of the tube was measured in millimeters. The inside walls of the tube were examined for evidence of wetting by the melted slag.

4 l determined amount of the flux into the furnace automatically at predetermined intervals of time. In some instances, as when the furnace is operated more or less intermittently, the flux may be added and the molten slag removed only just prior to shut down. Also, the sodium fluoride fluxing agent may advantageously be spread on the floor of the firebox, particularly in the vicinity of the tap hole of a wet-bottom furnace, just prior to putting the furnace into operation. In many instances it will be found necessary to add flux to the furnace only when it is about to be shut down for maintenance and repair in order to facilitate removal of accumulated slag. It will be apparent that the principle of the invention maybe applied in various ways depending upon the particular circumstances involved.

As previously stated, the composition and flow temperature of petroleum coke slag varies somewhat with the source of the coke. Accordingly, the amount of flux employed will vary somewhat according to the source of the coke. It may also vary somewhat with the rate at which the slag is formed and may further depend somewhat upon the type of furnace involved. In general, however, the amount of sodium fluoride employed Fluxing Agent ,Down" Down Top Distance Temp, Temp. Temp., of Flow, 11.

F. Lower- F. mm. Wetting Identity Percent ing, F.

None (Blank) 2,118 0 2, 375 8 None. Ferric Oxide. 10 2, 113 2, 375 7 Do, Do 20 2, 232 114 2, 375 7 Do. Do 40 I 2,375 257 2,375 Do. Calcium Fluoride- 2, 095 23 2,375 8 Do.

- D 20 2,136 18 2, 375 9 Some. D0. 40 2,167 -49 2, 375 Do. Do. 80 2,167 49 2, 375 Good.

Sodium Sulfate 20 2, 201 83 2, 435 20 Do. D0 40 2,131 13 2, 435 20 Bubbled over. 80 1, 843 275 2, 433 20 D0. Sodium Tetraborate 1,733 385 2, 383 I 20 Complete.

l, 674 444 2, 883 1 20 D0. 1, 609 509 2, 383 1 20 Do. Sodium Fluonde. 4 1, 990 128 2, 368 9 None. D 8 1, 890 228 2, 368 10 D0. Do. 16 1, 675 443 2, 368 15 Good. Do 24 600 518 2, 367 20 Do. Do. 32 1, 590 528 2, 379 20 Complete. D0. 40 1, 620 498 2, 365 20 D0. 130.. 51 1,620 498 2, 365 20 Do.

1 Did not melt at Top" temperature.

Slag and fiux completely volatllized.

It will be noted that of the various fluxing agents tested, sodium fluoride was the only one which was effective in reasonable proportions, and was capable of lowering the down temperature by more than 500 F. When tested by the Standard ASTM method, sodium fluoride in 10 percent concentration lowered the softening temperature by about 350 F. and at 2372 F. produced a highly fluid slagwhich readily spread out over the refractory base plate. It will also be noted that while sodium tetraborate was effective in lowering the down temperaturewhen used in relatively large amounts, it caused volatilization of {the slag. Such effect is highly undesirable since it results in deposition of the slag on the boiler tubes, flues, and other inaccessible places.

. Advantage may be taken of the fluxing effect of sodium fluoride on petroleum coke slag. in various ways. In some instances it is desirable that the sodium fluoride be more or less continuously introduced into the firebox in powdered form along with the petroleum coke fuel. The amount of sodium fluoride so employed will vary with the amount of slag produced by the fuel but will usually be of the order of 0.5 to 1.5 lbs. per ton of petroleum coke fuel. In other instances,-as where the coke produces only a relatively small amount of slag, the sodium fluoride may periodically be introduced into the firebox,'either manually by simply shoveling it in, or in admixture with the coke, or by use of mechanical equipment which may be adapted to introduce a prewill usually amount to between about 5 and about 40, preferably between about 20 and about 30 percent by weight of the combined slag and flux. Such proportions correspond to the use of between about 5 and about 70, preferably between about 25 and about 45, pounds of sodium fluoride per pounds of slag.

Other modes of applying the principle of my invention may be employed instead of those explained, change being made as regards the materials employed provided the step or steps stated by any of the following claims or the equivalent of such stated step or steps be employed.

I, therefore, particularly point out and distinctly claim as my invention:

1. The method of lowering the flow temperature of petroleum coke slag which comprises admixing therewith' an effective amount of sodium fluoride.

2. The method of lowering the flow temperature of petroleum coke slag which comprises admixing there with between about 5 and about 70 pounds of sodium fluoride per 100 pounds of said slag.

3. The method of lowering the flow temperature of petroleum coke slag which comprises admixing therewith between about 25 and about 45 pounds of sodium fluoride per 100 pounds of said slag.

4. In a process wherein petroleum coke is combusted in a firebox with the formation therein of a petroleum coke slag, the method of rendering said slag fluidatthe prevailing firebox temperature which comprises introducing into said firebox an effective amount of sodium fluoride and causing said soditm fluoride to come in contact with said slag.

5. The method of claim 4 wherein the sodium fluoride is employed in an amount representing between about 5 and about 70 pounds of sodium fluoride per 100 pounds of slag.

6. The method of claim 4 wherein the sodium fluoride is employed in an amount representing between about 25 and about 45 pounds of sodium fluoride per 100 pounds of slag.

7. The method of claim 6 wherein the sodium fluoride is introduced into the firebox in admixture with the petroleum coke fuel.

References Cited in the file of this patent UNITED STATES PATENTS Collian Feb. 1, 1887 Hitchcock Nov. 21, 1905 Schurecht May 11, 1926 Pacz Nov. 13, 1928 Weimer Apr. 18, 1944 FOREIGN PATENTS Switzerland Oct. 16, 1943 Great Britain Aug. 6, 1903 

1. THE METHOD OF LOWERING THE FLOW TEMPERATURE OF PETROLEUM COKE SLAG WHICH COMPRISES ADMIXING THEREWITH AN EFFECTIVE AMOUNT OF SODIUM FLUORIDE. 